US9204507B2 - LED lighting device - Google Patents

LED lighting device Download PDF

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Publication number
US9204507B2
US9204507B2 US14/370,166 US201214370166A US9204507B2 US 9204507 B2 US9204507 B2 US 9204507B2 US 201214370166 A US201214370166 A US 201214370166A US 9204507 B2 US9204507 B2 US 9204507B2
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Prior art keywords
current
led
circuit
flow
bypassing
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US14/370,166
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US20140368129A1 (en
Inventor
Shogo Fukuda
Tomoyuki Ichiza
Toshimasa Ogawa
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Sharp Corp
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Sharp Corp
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Priority claimed from JP2012014293A external-priority patent/JP5054236B1/ja
Priority claimed from JP2012196260A external-priority patent/JP5149457B1/ja
Priority claimed from JP2012196262A external-priority patent/JP5149458B1/ja
Application filed by Sharp Corp filed Critical Sharp Corp
Assigned to SHARP KABUSHIKI KAISHA reassignment SHARP KABUSHIKI KAISHA ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: OGAWA, Toshimasa, FUKUDA, SHOGO, ICHIZA, TOMOYUKI
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/30—Driver circuits
    • H05B45/37—Converter circuits
    • H05B45/3725—Switched mode power supply [SMPS]
    • H05B45/375—Switched mode power supply [SMPS] using buck topology
    • H05B33/0815—
    • H05B33/0818—
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/30—Driver circuits
    • H05B45/37—Converter circuits
    • H05B45/3725—Switched mode power supply [SMPS]
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/30—Driver circuits
    • H05B45/37—Converter circuits
    • H05B45/3725—Switched mode power supply [SMPS]
    • H05B45/38—Switched mode power supply [SMPS] using boost topology
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B20/00—Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps
    • Y02B20/30—Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]
    • Y02B20/343—
    • Y02B20/345—

Definitions

  • the present invention relates to an LED lighting device and, more particularly, to an LED lighting device to turn on a light emitting diode (LED) used in a backlight light source of a liquid crystal displaying apparatus.
  • LED light emitting diode
  • the LED is a semiconductor element that emits light when a voltage is applied thereto in the forward direction thereof.
  • a positive voltage and a negative voltage are respectively applied to the anode and the cathode of the LED, a current flows in the LED at a voltage of several volts and the LED emits light.
  • Control of turning on and off of, and control of the brightness of the backlight are enabled for each area by using the LEDs as the backlight light source and, therefore, the contrast can significantly be improved.
  • the LED has a longer life and needs a smaller power consumption than those of a traditional fluorescent tube and, therefore, has an advantage of a higher energy-saving effect than that thereof.
  • FIG. 7 is a diagram of the configuration of a traditional LED lighting device and “100” therein denotes the LED lighting device.
  • the LED lighting device 100 includes a converter circuit 2 including a switching power source 3 , an LED driving circuit 4 to drive LEDs ( 1 to 5 ) of an LED circuit 5 , and the LED circuit 5 including the plural LEDs ( 1 to 5 ).
  • the LED driving circuit 4 is mounted as an integrated circuit (IC) chip including a known microcomputer and peripheral circuits. An output of a signal from the LED driving circuit 4 to an external circuit is controlled by the microcomputer.
  • IC integrated circuit
  • the LED driving circuit 4 receives a dimming control signal from a main control portion (not depicted) to control a dimming duty ratio to a predetermined dimming duty ratio.
  • the LED driving circuit 4 outputs a duty signal based on the dimming duty ratio from its feedback output terminal to the converter circuit 2 , controls a start of the operation and discontinuation of the operation of the switching power source 3 , and causes the switching power source 3 to generate a voltage V 1 corresponding to the dimming duty ratio.
  • the LED driving circuit 4 causes a switching element TR 1 inserted between the LED circuit 5 and a ground line GND to be driven for switching based on the duty signal; causes the voltage V 1 to continuously be applied to the LED circuit 5 ; and, thereby, causes the LEDs ( 1 to 5 ) to be turned on.
  • the switching element TR 1 is composed of, for example, a field effect transistor (FET).
  • the converter circuit 2 generates the voltage V 1 to turn on the LEDs ( 1 to 5 ) included in the LED circuit 5 based, for example, on a reference voltage V generated from the power source voltage (+B) such as a battery.
  • a reference voltage V generated from the power source voltage (+B) such as a battery.
  • the converter circuit 2 is configured as a what-is-called step-down converter including a capacitor C, the converter circuit 2 may be of any one of a step-down, a step-up, and a step-up-and-down types.
  • the LED circuit 5 includes one or more LEDs.
  • the LED circuit 5 includes five LEDs of LEDs ( 1 to 5 ).
  • An electric current i flowing in the LED circuit 5 is converted from a current to a voltage by a current detection resistor R 1 and the voltage is input into a current detection terminal of the LED driving circuit 4 .
  • the LED driving circuit 4 outputs the value of the voltage input into the current detection terminal from its feedback output terminal to the converter circuit 2 , and causes the converter circuit 2 to drive the LEDs ( 1 to 5 ) each with a constant current.
  • the contrast ratio is determined based on the ratio of the maximal luminance and the minimal luminance of the LEDs.
  • An increase of the contrast ratio is equivalent to an increase of the ratio of the maximal current: (the rated current) and the minimal current that flow in the LEDs.
  • the current flowing in the LEDs is determined by a current supplied by the LED driving circuit as described with reference to FIG. 7 .
  • the manufacturers of the liquid crystal displaying apparatuses each set target ratio of the maximal current to the minimal current to be caused to flow in the LEDs, to realize the desired contrast ratio.
  • the target ratio is set to be 2000:1.
  • the target value of the minimal current is 0.125 mA.
  • LED minimal current Two methods are traditionally used to realize the minimal current to be caused to flow in the LEDs (hereinafter, referred to as “LED minimal current”).
  • a method is present as the first method according to which a main control portion adjusts a dimming control signal (the dimming duty) and the LED driving circuit generates the LED minimal current based on the dimming duty after the adjustment.
  • the main control portion needs to execute complicated signal adjustment, and a problem arises that the performance of the LED driving circuit, that is, the range of the dimming duty that the LED driving circuit can handle restricts the LED minimal current. This will be described with reference to FIG. 8 .
  • FIG. 8 is a diagram of the relation between the current flowing in the LED and the dimming duty.
  • the axis of ordinate therein represents the current flowing in the LED (unit: mA) and the axis of abscissa therein represents the dimming duty (%).
  • a graph L 1 shows the correspondence relation between the dimming duty and the current flowing in the LED (that is, the current supplied by the LED driving circuit).
  • the dimming duty when the dimming duty is equal to or lower than 0.1% (in FIG. 8 , in a range indicated by “X”), it can be seen that: the operation is not guaranteed due to the performance of the LED driving circuit; and, therefore, the target 0.125 mA cannot be achieved.
  • the dimming duty is 0.1%
  • the current flowing in the LED is 0.200 mA and, therefore, the target 0.125 mA cannot be achieved.
  • Another method is present as the second method according to which an external circuit of the LED driving circuit realizes the LED minimal current.
  • the LED driving circuit is caused to discontinue its operation and the external circuit generates the LED minimal current and, therefore, the LED driving circuit may malfunction when the LED driving circuit recovers its operation.
  • the LED driving circuit whose operation is discontinued is caused to precipitously recover its operation, the LED voltage is increased. Therefore, this increase of the LED voltage is mistakenly detected as an excess voltage error.
  • the present invention was conceived in view of the above circumstances and an object thereof is to provide an LED lighting device that can set the ratio of the maximal current to the minimal current caused to flow in the LED to be the target ratio regardless of the performance of the LED driving circuit and without any malfunctioning thereof.
  • a first technical means of the present invention is an LED lighting device comprising: an LED circuit that comprises at least one LED; a bypassing circuit that is inserted in parallel to the LED ciruit; and an LED driving circuit that supplies a current to each of the LED circuit and the bypassing circuit, wherein when a predetermined current is caused to flow in the LED circuit, the LED driving circuit supplies a current acquired by adding the current to flow in the bypassing circuit to the predetermined current, and wherein the current acquired by the addition is controlled by a switching circuit connected in series to the LED circuit.
  • a second technical means is the LED lighting device comprising: an LED circuit that comprises at least one LED; a bypassing circuit that is inserted in parallel to the LED circuit; and and LED driving circuit that supplies a current to each of the LED circuit and the bypassing circuit, wherein when a predetermined current is caused to flow in the LED circuit, the LED driving circuit supplies a current acquired by adding the current to flow in the bypassing circuit to the predetermined current, and wherein the current acquired by the addition is controlled by a switching circuit connected in series to the LED circuit and the bypassing circuit.
  • a third technical means is the LED lighting device of the second technical means, wherein when a predetermined maximal current is caused to flow in the LED circuit, the LED driving circuit supplies a current acquired by adding the current to flow in the bypassing circuit to the predetermined maximal current.
  • a fourth technical means is the LED lighting device of the second technical means, wherein the bypassing circuit is a resistor.
  • a fifth technical means is the LED lighting device of the second technical means, wherein the bypassing circuit comprises at least one bypass line having therein a resistor and another switching circuit connected in series to each other, and wherein when the predetermined current is caused to flow in the LED circuit, the other switching circuit is caused to operate and the LED driving circuit supplies a current acquired by adding the current to flow in the bypassing circuit to the predetermined current.
  • a sixth technical means is the LED lighting device of the fifth technical means, wherein when a predetermined maximal current is caused to flow in the LED circuit, the other switching circuit is caused to stop operation and the LED driving circuit supplies the predetermined maximal current.
  • a seventh technical means is the LED lighting device of the fifth technical means, wherein the bypassing circuit comprises the plurality of bypass lines connected in parallel to each other.
  • An eighth technical means is the LED lighting device of the second technical means, wherein the bypassing circuit comprises a semiconductor element, and wherein when the predetermined current is caused to flow in the LED circuit, the semiconductor element is caused to operate and the LED driving circuit supplies a current acquired by adding the current to flow in the bypassing circuit to the predetermined current.
  • a ninth technical means is the LED lighting device of the eighth technical means, wherein when the predetermined maximal current is caused to flow in the LED circuit, the semiconductor element is caused to stop operation and the LED driving circuit supplies the predetermined maximal current.
  • a tenth technical means is the LED lighting device of eighth technical means, wherein the semiconductor element is a field-effect transistor, and wherein the current to flow in the bypassing circuit is controlled by controlling a gate-source voltage of the field-effect transistor.
  • An eleventh technical means is the LED lighting device of the second technical means, wherein the predetermined maximal and the predetermined mininmal currents to be caused to flow in the LED circuit have a predetermined ratio.
  • a twelfth technical means is the LED lighting device of the second technical means, wherein the LED driving circuit supplies a current through duty control.
  • a bypassing circuit is inserted in parallel to an LED and, thereby, the current supplied by the LED driving circuit can be branched to the bypassing circuit such that the current caused to flow in the LED becomes the target minimal current. Therefore, the ratio of the maximal current to the minimal current caused to flow in the LED can be set to be the target ratio regardless of the performance of the LED driving circuit and without any malfunctioning of the LED driving circuit because the operation thereof is not discontinued.
  • FIG. 1 is a diagram of an exemplary configuration of an LED lighting device according to a first embodiment of the present invention.
  • FIG. 2 is a diagram of an example of a correspondence relation between a current flowing in the LED and a dimming duty.
  • FIG. 3 is a diagram of an exemplary configuration of an LED lighting device according to a second embodiment of the present invention.
  • FIG. 4 is a diagram of an exemplary configuration of an LED lighting device according to a third embodiment of the present invention.
  • FIG. 5 is a diagram of another example of the correspondence relation between the current flowing in the LED and the dimming duty.
  • FIG. 6 is a diagram of an example of a control function expressing the relation between a gate-source voltage and a current of a semiconductor device.
  • FIG. 7 is a diagram of a configuration of a traditional LED lighting device.
  • FIG. 8 is a diagram of the relation between the current flowing in the LED and the dimming duty.
  • the LED lighting device is incorporated in a liquid crystal displaying apparatus using LEDs as its backlight light source and executes lighting control of the LEDs.
  • FIG. 1 is a diagram of an exemplary configuration of an LED lighting device according to a first embodiment of the present invention.
  • “ 1 a ” denotes the LED lighting device.
  • the LED lighting device 1 a differs from the LED lighting device 100 described as above with reference to FIG. 7 in that the LED lighting device 1 a includes a bypassing circuit 6 a inserted in parallel to the LED circuit 5 .
  • components denoted by the same reference numerals as those of the components depicted in FIG. 7 have the same functions as those thereof.
  • An exemplary operation of the LED lighting device 1 a will briefly be described.
  • the LED driving circuit 4 receives the dimming control signal to control the dimming duty ratio to the predetermined dimming duty ratio, from the main control portion (not depicted).
  • the LED driving circuit 4 outputs the duty signal based on the dimming duty ratio from its feedback output terminal to the converter circuit 2 , controls turning on and off of the switching power source 3 , and causes the switching power source 3 to generate the voltage V 1 corresponding to the dimming duty ratio.
  • the LED driving circuit 4 causes the switching element TR 1 (corresponding to a switching circuit) inserted between the LED circuit 5 and the ground line GND to be driven for switching based on the duty signal, causes the voltage V 1 to continuously be applied to the LED circuit 5 , and causes the LEDs ( 1 to 5 ) to be turned on.
  • the switching element TR 1 is composed of, for example, a field effect transistor (FET).
  • the converter circuit 2 generates the voltage V 1 to turn on the LEDs ( 1 to 5 ) included in the LED circuit 5 based, for example, on the reference voltage V generated from the power source voltage (+B) such as a battery.
  • the converter circuit 2 is configured as a what-is-called step-down converter including a capacitor C, the converter circuit 2 may be of any one of a step-down, a step-up, and a step-up-and-down types.
  • the LED circuit 5 includes at least one LED.
  • the LED circuit 5 includes the five LEDs of LEDs ( 1 to 5 ).
  • the electric current i acquired by adding an electric current i LED flowing in the LEDs ( 1 to 5 ) to an electric current i R flowing in the bypassing circuit 6 a is converted from a current into a voltage by the current detection resistor R 1 and this voltage is input into the current detection terminal of the LED driving circuit 4 .
  • the LED driving circuit 4 outputs the value of the voltage input into the current detection terminal from its feedback output terminal to the converter circuit 2 , and causes the converter circuit 2 to drive the LEDs ( 1 to 5 ) each with a constant current.
  • a primary object of the present invention is to set the ratio of the maximal current to the minimal current to be caused to flow in the LEDs, to be the target ratio regardless of the performance of the LED driving circuit and without any malfunctioning thereof.
  • the LED lighting device 1 a includes, as the configuration to achieve the object, the LED circuit 5 including the LEDs ( 1 to 5 ), the bypassing circuit 6 a inserted in parallel to the LED circuit 5 , and the LED driving circuit 4 to supply an electric current to each of the LED circuit 5 and the bypassing circuit 6 a based, for example, on the duty control.
  • the LED driving circuit 4 causes a predetermined minimal current to flow in the LED circuit 5
  • the LED driving circuit 4 supplies an electric current acquired by adding the current to be caused to flow in the bypassing circuit 6 a to this minimal current.
  • the electric current acquired by the addition is controlled by the switching element TR 1 connected in series to the LED circuit 5 and the bypassing circuit 6 a.
  • the current supplied by the LED driving circuit 4 is branched to the bypassing circuit 6 a such that the value of the current to be caused to flow in the LEDs ( 1 to 5 ) becomes the target value that is the value of the minimal current acquired when the ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) becomes predetermined ratio (also referred to as “target ratio”).
  • the target ratio only has to be determined corresponding to the contrast ratio desired to be realized, the specification of the LED, etc., and is not especially limited while the target ratio may be, for example, 2000:1.
  • the LED driving circuit 4 controls turning on and off of the switching power source 3 based on the externally-input dimming control signal (the dimming duty ratio), and supplies the current i based on the voltage V 1 that corresponds to the dimming duty ratio.
  • the current i is branched at the connection point of the LEDs ( 1 to 5 ) and a resistor R 2 .
  • the currents i LED and i R respectively flow in the LEDs ( 1 to 5 ) and the resistor R 2 .
  • the LED driving circuit 4 supplies the current based on the duty control is exemplified and described. However, not limited to this duty control, the same control can be executed even in the case, for example, where current control is applied.
  • the resistor R 2 is inserted in parallel to the LEDs ( 1 to 5 ) as an example of the bypassing circuit 6 a .
  • the bypassing circuit 6 a is not limited to the resistor R 2 but may be an element having an impedance component and, for example, a thermistor or a transistor may be used.
  • the target ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) is 2000:1 and the maximal current (the rated current) is 250 mA.
  • the target value of the minimal current is 0.125 mA.
  • the performance of the LED driving circuit 4 in this example as depicted in FIG. 8 , the operation thereof is not guaranteed when the dimming duty is equal to or lower than 0.1%, and 0.200 mA is supplied to the LEDs ( 1 to 5 ) when the dimming duty is, for example, 0.1%. With this performance of the LED driving circuit 4 , the target value of the minimal current that is 0.125 mA cannot be achieved.
  • the current i R to flow in the resistor R 2 is 0.075 mA. Based on this, the resistance value of the resistor R 2 can be acquired from V 1 /i R .
  • the voltage V 1 is the voltage applied across the resistor R 2 (and the LED circuit 5 ) and is known.
  • the LED driving circuit 4 When the LED driving circuit 4 causes the maximal current (for example, 250 mA) to flow in the LEDs ( 1 to 5 ), the LED driving circuit 4 only has to supply a current acquired by adding the current i R to be caused to flow in the resistor R 2 , to the maximal current.
  • the resistance value of the resistor R 2 can be determined from the above.
  • a voltage V 1 ′ is determined in advance that is necessary for causing the maximal current (250 mA) to flow in the LEDs ( 1 to 5 ) (the dimming duty ratio).
  • the current i R can be determined from the resistor R 2 and the voltage V 1 ′.
  • FIG. 2 is a diagram of an example of the correspondence relation between the current flowing in the LEDs and the dimming duty.
  • the axis of ordinate represents the current (unit: mA) flowing in the LEDs and the axis of abscissa represents the dimming duty (%).
  • the graph L 1 is the same as that for the traditional case described above with reference to FIG. 8 .
  • a graph L 2 shows the correspondence relation between the current flowing in the LEDs ( 1 to 5 ) and the dimming duty in the circuit configuration of this embodiment, and is acquired by shifting the graph L 1 to be lower by the amount of the current i R .
  • the ratio of the maximal current to the minimal current of the current i LED to be caused to flow in the LEDs can be set to be the target ratio (for example, 2000:1).
  • the insertion of the bypassing circuit 6 a in parallel to the LEDs ( 1 to 5 ) enables the current i supplied by the LED driving circuit 4 to be branched to the bypassing circuit 6 a such that the current i LED to be caused to flow in the LEDs ( 1 to 5 ) becomes the target minimal current. Therefore, the ratio of the maximal current to the minimal current caused to flow in the LEDs ( 1 to 5 ) can be set to be the target ratio regardless of the performance of the LED driving circuit 4 and without any malfunctioning of the LED driving circuit 4 because the operation thereof is not discontinued.
  • the resistor R 2 When the resistor R 2 is used as the bypassing circuit 6 a , the resistor R 2 also functions as a discharge resistance for an abnormal situation and, therefore, the circuit can be protected and the safety can be secured of the LED lighting device 1 a or the liquid crystal displaying apparatus incorporating therein the LED lighting device 1 a.
  • FIG. 3 is a diagram of an exemplary configuration of an LED lighting device according to a second embodiment of the present invention.
  • “ 1 b ” denotes the LED lighting device.
  • the LED lighting device 1 b differs from the LED lighting device 100 described above with reference to FIG. 7 in that the LED lighting device 1 b includes a bypassing circuit 6 b inserted in parallel to the LED circuit 5 and a control circuit 7 that controls turning on and off of the bypassing circuit 6 b .
  • components denoted by the same reference numerals as those of the components depicted in FIG. 7 have the same functions as those thereof. An exemplary operation of the LED lighting device 1 b will briefly be described.
  • the LED driving circuit 4 receives the dimming control signal to control the dimming duty ratio to the predetermined dimming duty ratio, from the main control portion (not depicted).
  • the LED driving circuit 4 outputs the duty signal based on the dimming duty ratio from its feedback output terminal to the converter circuit 2 , controls turning on and off of the switching power source 3 , and causes the switching power source 3 to generate the voltage V 1 corresponding to the dimming duty ratio.
  • the LED driving circuit 4 causes the switching element TR 1 (corresponding to the switching circuit) inserted between the LED circuit 5 and the ground line GND to be driven for switching based on the duty signal, causes the voltage V 1 to continuously be applied to the LED circuit 5 , and causes the LEDs ( 1 to 5 ) to be turned on.
  • the switching element TR 1 is composed of, for example, a field effect transistor (FET).
  • the converter circuit 2 generates the voltage V 1 to turn on the LEDs ( 1 to 5 ) included in the LED circuit 5 based, for example, on the reference voltage V generated from the power source voltage (+B) such as a battery.
  • the converter circuit 2 is configured as a what-is-called step-down converter including the capacitor C, the converter circuit 2 may be of any one of a step-down, a step-up, and a step-up-and-down types.
  • the LED circuit 5 includes at least one LED.
  • the LED circuit 5 includes the five LEDs of LEDs ( 1 to 5 ).
  • the current i acquired by adding an electric current i LED flowing in the LEDs ( 1 to 5 ) to an electric current i R flowing in the bypassing circuit 6 b is converted from a current into a voltage by the current detection resistor R 1 and this voltage is input into the current detection terminal of the LED driving circuit 4 .
  • the LED driving circuit 4 outputs the value of the voltage input into the current detection terminal from its feedback output terminal to the converter circuit 2 , and causes the converter circuit 2 to drive the LEDs ( 1 to 5 ) each with a constant current.
  • the LED lighting device 1 b of this embodiment includes the LED circuit 5 including the LEDs ( 1 to 5 ), the bypassing circuit 6 b inserted in parallel to the LED circuit 5 , and the LED driving circuit 4 that supplies a current to the LED circuit 5 and the bypassing circuit 6 b based, for example, on the duty control.
  • the LED driving circuit 4 causes a predetermined minimal current to flow in the LED circuit 5
  • the LED driving circuit 4 supplies a current acquired by adding the current to flow in the bypassing circuit 6 b to this minimal current.
  • the current acquired by the addition is controlled by the switching element TR 1 connected in series to the LED circuit 5 and the bypassing circuit 6 b.
  • the current supplied by the LED driving circuit 4 is branched to the bypassing circuit 6 b such that the value of the current to be caused to flow in the LEDs ( 1 to 5 ) becomes the target value that is the value of the minimal current acquired when the ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) become predetermined ratio (also referred to as “target ratio”).
  • the target ratio only has to be determined corresponding to the contrast ratio desired to be realized, the specification of the LED, etc., and is not especially limited while the target ratio may be, for example, 2000:1.
  • the LED driving circuit 4 controls turning on and off of the switching power source 3 based on the externally-input dimming control signal (the dimming duty ratio), and supplies the current i based on the voltage V 1 that corresponds to the dimming duty ratio.
  • the current i is branched at the connection point of the LEDs ( 1 to 5 ) and the bypassing circuit 6 b .
  • the currents i LED and i R respectively flow in the LEDs ( 1 to 5 ) and the bypassing circuit 6 b .
  • the LED driving circuit 4 supplies the current based on the duty control is exemplified and described. However, not limited to this duty control, the same control can be executed even in the case, for example, where current control is applied.
  • the bypassing circuit 6 b includes at least one bypass line having therein a resistor and a switching element connected to each other in series.
  • the bypassing circuit 6 b includes a bypass line having therein a resistor R A and a switching element T A connected to each other in series, a bypass line having therein a resistor R B and a switching element T B connected to each other in series, and a bypass line having therein a resistor R C and a switching element T C connected to each other in series; and has therein these three bypass lines connected in parallel to each other.
  • the switching elements T A to T C each correspond to another switching circuit, and are each composed of, for example, an field-effect transistor (FET) or, more preferably, a metal-oxide-semiconductor filed-effect transistor (MOSFET).
  • FET field-effect transistor
  • MOSFET metal-oxide-semiconductor filed-effect transistor
  • the control circuit 7 is connected to the switching elements T A to T C and controls turning on and off (operation and discontinuation of the operation) of each of the switching elements T A to T C .
  • i LED i ⁇ (i A +i B +i C ) from Eq. (2).
  • the target ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) is 2000:1 and the maximal current (the rated current) is 250 mA.
  • the target value of the minimal current is 0.125 mA.
  • the operation is not guaranteed when the dimming duty is equal to or lower than 0.1%, and 0.200 mA is supplied to the LEDs ( 1 to 5 ) when the dimming duty is, for example, 0.1%.
  • the target value of the minimal current that is 0.125 mA cannot be achieved.
  • the current i C to flow in the resistor R C is 0.075 mA.
  • the resistance value of the resistor R C can be acquired from V 1 /i C .
  • the voltage V 1 is the voltage applied across the resistor R C (and the LED circuit 5 ) and is known.
  • the minimal current is adapted to flow in the LEDs ( 1 to 5 ) by executing the control to turn off the switching elements T A and T B and to turn on the switching element T C .
  • the minimal current may be adapted to flow in the LEDs ( 1 to 5 ) by executing control to turn on any one or more of the switching elements T A , T B , and T C .
  • the maximal current for example, 250 mA
  • control is executed to turn off all of the switching elements T A to T C .
  • the bypassing circuit 6 b When the bypassing circuit 6 b is composed only of the resistors, a current always flows in the bypassing circuit 6 b . Therefore, even when the maximal current is caused to flow in the LEDs ( 1 to 5 ), the LED driving circuit 4 needs to supply a current acquired by adding a current to be supplied to the bypassing circuit 6 b to the maximal current and, the added current is lost.
  • the bypassing circuit 6 b is composed of the resistors and the switching elements. Therefore, when the maximal current is caused to flow in the LEDs ( 1 to 5 ), no current can be caused to flow in the resistors by turning off the switching elements. Thereby, the LED driving circuit 4 only has to supply the maximal current as it is and, therefore, the above loss does not occur.
  • the correspondence relation between the current flowing in the LEDs and the dimming duty will be described with reference to FIG. 2 .
  • the graph L 1 is the same as that for the traditional case described above with reference to FIG. 8 .
  • the ratio of the maximal current to the minimal current of the current i LED to be caused to flow in the LEDs ( 1 to 5 ) can be set to be the target ratio (for example, 2000:1).
  • the desired current ratio (ratio of the maximal current to the minimal current) can be realized by connecting the plural bypass lines in parallel to each other and controlling the combination of turning on and turning off for the switching elements T A to T C .
  • the pattern of (2) is employed.
  • at least the bypass line including the switching element T A only has to be present, and the bypass line including the switching element T B and the bypass line including the switching element T C may be unnecessary.
  • the insertion of the bypassing circuit 6 b in parallel to the LEDs ( 1 to 5 ) enables the current i supplied by the LED driving circuit 4 to be branched to the bypassing circuit 6 b such that the current i LED to be caused to flow in the LEDs ( 1 to 5 ) becomes the target minimal current. Therefore, the ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) can be set to be the target ratio regardless of the performance of the LED driving circuit 4 and without any malfunctioning of the LED driving circuit 4 because the operation thereof is not discontinued.
  • bypassing circuit 6 b including a resistance is used and the resistance works as a discharge resistance for an abnormal situation and, therefore, the circuit can be protected and the safety can be secured of the LED lighting device 1 b or the liquid crystal displaying apparatus incorporating therein the LED lighting device 1 b.
  • FIG. 4 is a diagram of an exemplary configuration of an LED lighting device according to a third embodiment of the present invention.
  • “ 1 c ” denotes the LED lighting device.
  • the LED lighting device 1 c differs from the LED lighting device 100 described above with reference to FIG. 7 in that the LED lighting device 1 c includes a bypassing circuit 6 c inserted in parallel to the LED circuit 5 and the control circuit 7 ′ that controls the operation of the bypassing circuit 6 c .
  • components denoted by the same reference numerals as those of the components depicted in FIG. 7 have the same functions as those thereof. An exemplary operation of the LED lighting device 1 c will briefly be described.
  • the LED driving circuit 4 receives the dimming control signal to control the dimming duty ratio to the predetermined dimming duty ratio, from the main control portion (not depicted).
  • the LED driving circuit 4 outputs the duty signal based on the dimming duty ratio from its feedback output terminal to the converter circuit 2 , controls turning on and off of the switching power source 3 , and causes the switching power source 3 to generate the voltage V 1 corresponding to the dimming duty ratio.
  • the LED driving circuit 4 causes the switching element TR 1 (corresponding to the switching circuit) inserted between the LED circuit 5 and the ground line GND to be driven for switching based on the duty signal, causes the voltage V 1 to continuously be applied to the LED circuit 5 , and causes the LEDs ( 1 to 5 ) to be turned on.
  • the switching element TR 1 is composed of, for example, a field effect transistor (FET).
  • the converter circuit 2 generates the voltage V 1 to turn on the LEDs ( 1 to 5 ) included in the LED circuit 5 based, for example, on the reference voltage V generated from the power source voltage (+B) such as a battery.
  • the converter circuit 2 is configured as a what-is-called step-down converter including the capacitor C, the converter circuit 2 may be of any one of a step-down, a step-up, and a step-up-and-down types.
  • the LED circuit 5 includes at least one LED.
  • the LED circuit 5 includes the five LEDs of LEDs ( 1 to 5 ).
  • the electric current i acquired by adding the current i LED flowing in the LEDs ( 1 to 5 ) to the current i M flowing in the bypassing circuit 6 c is converted from a current into a voltage by the current detection resistor R 1 and this voltage is input into the current detection terminal of the LED driving circuit 4 .
  • the LED driving circuit 4 outputs the value of the voltage input into the current detection terminal from its feedback output terminal to the converter circuit 2 , and causes the converter circuit 2 to drive the LEDs ( 1 to 5 ) each with a constant current.
  • the LED lighting device 1 c of this embodiment includes the LED circuit 5 including the LEDs ( 1 to 5 ), the bypassing circuit 6 c inserted in parallel to the LED circuit 5 , and the LED driving circuit 4 that supplies a current to each of the LED circuit 5 and the bypassing circuit 6 c based, for example, on the duty control.
  • the LED driving circuit 4 causes the predetermined minimal current to flow in the LED circuit 5
  • the LED driving circuit 4 supplies the current acquired by adding the current caused to flow in the bypassing circuit 6 c to this minimal current.
  • the current acquired by the addition is controlled by the switching element TR 1 connected in series to the LED circuit 5 and the bypassing circuit 6 c.
  • the current supplied by the LED driving circuit 4 is branched to the bypassing circuit 6 c such that the value of the current to be caused to flow in the LEDs ( 1 to 5 ) becomes the target value that is the value of the minimal current acquired when the ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) become the predetermined ratio (also referred to as “target ratio”).
  • the target ratio only has to be determined corresponding to the contrast ratio desired to be realized, the specification of the LED, etc., and is not especially limited while the target ratio may be, for example, 2000:1.
  • the LED driving circuit 4 controls turning on and off of the switching power source 3 based on the externally-input dimming control signal (the dimming duty ratio), and supplies the current i based on the voltage V 1 that corresponds to the dimming duty ratio.
  • the current i is branched at the connection point of the LEDs ( 1 to 5 ) and the bypassing circuit 6 c .
  • the currents i LED and i M respectively flow in the LEDs ( 1 to 5 ) and the bypassing circuit 6 c .
  • the LED driving circuit 4 supplies the current based on the duty control is exemplified and described. However, not limited to this duty control, the same control can be executed even in the case, for example, where current control is applied.
  • the bypassing circuit 6 c is composed of: a semiconductor element such as a field-effect transistor (FET) or, more preferably, a metal-oxide-semiconductor field-effect transistor (MOSFET) that is a type of FET; etc.
  • FET field-effect transistor
  • MOSFET metal-oxide-semiconductor field-effect transistor
  • the control circuit 7 ′ is connected to the FET element T M , and controls turning on and off (operation and discontinuation of the operation) of the FET element T M and the gate-source voltage V GS to be applied to the FET element T M .
  • the current i M flowing in the bypassing circuit 6 c is controlled by the gate-source voltage V GS of the FET element T M .
  • the target ratio of the maximal current to the minimal current to be caused to flow in the LEDs ( 1 to 5 ) is 2000:1 and the maximal current (the rated current) is 250 mA.
  • the target value of the minimal current is 0.125 mA.
  • the operation thereof is not guaranteed when the dimming duty is equal to or lower than 0.1%, and 0.200 mA is supplied to the LEDs ( 1 to 5 ) when the dimming duty is, for example, 0.1%.
  • the target value of the minimal current that is 0.125 mA cannot be achieved.
  • the current i M to be caused to flow in the FET element T M is 0.075 mA.
  • the gate-source voltage V GS necessary for causing this current i M to flow can be acquired based on a control function described with reference to FIG. 6 .
  • the voltage V 1 is the voltage applied across the FET element T M (and the LED circuit 5 ) and is known.
  • the maximal current for example, 250 mA
  • control is executed to turn off the FET element T M .
  • the bypassing circuit 6 c is composed only of resistors, a current always flows in the bypassing circuit 6 c . Therefore, even when the maximal current is caused to flow in the LEDs ( 1 to 5 ), the LED driving circuit 4 needs to supply a current acquired by adding a current to be supplied to the bypassing circuit 6 c to the maximal current and, the added current is lost.
  • the bypassing circuit 6 c is composed of the FET element. Therefore, when the maximal current is caused to flow in the LEDs ( 1 to 5 ), no current can be caused to flow in the FET element by turning off the FET element. Thereby, the LED driving circuit 4 only has to supply the maximal current as it is and, therefore, the above loss does not occur.
  • FIG. 5 is a diagram of another example of the correspondence relation between the current flowing in the LEDs and the dimming duty.
  • the axis of ordinate therein represents the current flowing in the LEDs (unit: mA) and the axis of abscissa therein represents the dimming duty (%).
  • the graph L 1 is the same as that for the traditional case described above with reference to FIG. 8 .
  • a graph L 2 ′ shows the correspondence relation between the current flowing in the LEDs ( 1 to 5 ) and the dimming duty in the circuit configuration of this embodiment, and is acquired by shifting the graph L 1 to be lower by the amount of the current i M .
  • the ratio of the maximal current to the minimal current of the current i LED to be caused to flow in the LEDs ( 1 to 5 ) can be set to be the target ratio (for example, 2000:1).
  • FIG. 6 is a diagram of an example of the control function expressing the relation between the gate-source voltage and the current of a semiconductor device. Another example of this embodiment will be described with reference to FIG. 4 .
  • the maximal current of the current i LED flowing in the LEDs ( 1 to 5 ) is, for example, 200 mA and the minimal current controllable by the LED driving circuit 4 is, for example, 1 mA.
  • the target ratio of the maximal current to the minimal current of the current i LED to be caused to flow in the LEDs ( 1 to 5 ) is 2000:1.
  • the maximal current is 200 mA and, therefore, the minimal current to be caused to flow in the LEDs ( 1 to 5 ) is 0.1 mA.
  • the control circuit 7 ′ applies 1.4 V as the gate-source voltage V GS acquired as above, to the FET element T M .
  • the current i M of 0.9 mA flows in the FET element T M .
  • the desired current ratio (the ratio of the maximal current to the minimal current) such as 2000:1 can be realized by controlling the gate-source voltage of the FET element T M .
  • the insertion of the bypassing circuit 6 c in parallel to the LEDs ( 1 to 5 ) enables the current i supplied by the LED driving circuit 4 to be branched to the bypassing circuit 6 c such that the current i LED to be caused to flow in the LEDs ( 1 to 5 ) becomes the target minimal current. Therefore, the ratio of the maximal current to the minimal current caused to flow in the LEDs ( 1 to 5 ) can be set to be the target ratio regardless of the performance of the LED driving circuit 4 and without any malfunctioning of the LED driving circuit 4 because the operation thereof is not discontinued.

Landscapes

  • Circuit Arrangement For Electric Light Sources In General (AREA)
  • Led Devices (AREA)
US14/370,166 2012-01-26 2012-09-21 LED lighting device Expired - Fee Related US9204507B2 (en)

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JP2012-014293 2012-01-26
JP2012014293A JP5054236B1 (ja) 2012-01-26 2012-01-26 Ｌｅｄ点灯装置
JP2012-196262 2012-09-06
JP2012-196260 2012-09-06
JP2012196260A JP5149457B1 (ja) 2012-09-06 2012-09-06 Ｌｅｄ点灯装置
JP2012196262A JP5149458B1 (ja) 2012-09-06 2012-09-06 Ｌｅｄ点灯装置
PCT/JP2012/074203 WO2013111377A1 (ja) 2012-01-26 2012-09-21 Ｌｅｄ点灯装置

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KR20160060232A (ko) * 2014-11-19 2016-05-30 삼성디스플레이 주식회사 백라이트 유닛

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US20070159750A1 (en) * 2006-01-09 2007-07-12 Powerdsine, Ltd. Fault Detection Mechanism for LED Backlighting
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JP2010257718A (ja) 2009-04-23 2010-11-11 Toshiba Lighting & Technology Corp Ｌｅｄ点灯装置及び標識灯システム
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